The present invention relates to a method for the surface coating of an interior fitting for vehicles, for example of a shaped wooden part, with a lacquer or resin layer, in which the interior fitting to be coated is inserted into a cavity of a molding tool, the inner wall of which is at a defined distance from the interior fitting in the region of the interior fitting surface to be coated, in particular in the region of a visible side of the interior fitting, and thereby forms, between the inner wall and the surface to be coated, a clearance extending over the entire surface to be coated and having a substantially constant thickness, and in which a liquid surface coating material is introduced into the clearance and is cured there.
Interior fittings of this type, with a lacquered surface, are used, for example, for fitting out the interior of motor vehicles. In this context, in particular, shaped wooden parts are employed, which may take the form of solid components or veneered components, the latter usually being provided on their rear side with a reinforcing element composed, for example, of plastic. A transparent lacquer or resin layer is applied to the visible side of the interior fittings, so that a usually glossy and hard-wearing surface is obtained and, in the case of shaped wooden parts with a transparent surface coating, the structure of the wood not only remains visible, but is additionally emphazised. For this purpose, the shaped wooden part is conventionally provided with a lacquer layer having a thickness of approximately 0.5 mm to about 0.8 mm, since this results in a visually attractive xe2x80x9cdepth effectxe2x80x9d.
In conventional production methods, surface coating materials employed as lacquers or resins are sprayed or painted onto the surfaces to be coated. In this case, in general, a plurality of layers are necessary, in order to acquire, on the one hand, the desired sealing and, on the other hand, also a surface which is as glossy as possible. So that the surfaces are free of inclusions and completely uniform, it is necessary to take enhanced precautionary measures. Moreover, spraying or painting makes it necessary, as a rule, to carry out an after-treatment. Thus, for example, in order to achieve the depth effect initially mentioned, it is necessary for lacquer layers to be applied successively in a plurality of steps until a layer thickness of 0.8 mm is achieved. The lacquer layer is then reduced again, by grinding and polishing, to a layer thickness of approximately 0.5 mm which then satisfies the desired visual requirements.
Instead of spraying or painting the surface coating materials onto the interior fitting, it has already taken proposed in DE 43 20 893 C1 to insert the interior fitting into a molding tool, the inner wall of which is at a slight distance from the interior fitting in the region of the interior fitting surface to be coated and thereby forms, between the inner wall and the surface, a clearance extending over the entire surface to be coated and having a substantially constant thickness. Subsequently, in the known method, the liquid surface coating material is introduced into the clearance and is then cured there. In order to ensure that there are no gas bubbles embedded in the surface layer, in the method known from DE 43 20 893 C2 the clearance is evacuated and the liquid surface coating material is introduced into the clearance during evacuation or thereafter.
It is known from U.S. Pat. No. 4,647,274 to coat an interior fitting for vehicles in a molding tool, in such a way that, in the region of the interior fitting surface to be coated, a liquid surface coating material having two components is introduced, which is cured in the molding tool under pressure and heat load.
In many cases, a satisfactory surface quality is achieved by means of the known method. It was shown, however, that a lacquer layer satisfying the most stringent requirements cannot be produced in all instances.
The object of the present invention is to develop the generic method in such a way that the quality of the surface coating is improved as far as possible without additional after-working.
In a method of the type initially described, this object is achieved, according to the invention, in that, after the clearance has been filled, the molding tool is separated from the mixing and feeding assembly and the surface coating material in the clearance is cured substantially under atmospheric pressure.
The method according to the invention employs as surface coating material a mixture of at least two components which are mixed with one another only immediately before the filling of the clearance. The surface coating material is introduced into the clearance by means of the feeding assembly and the mixing and feeding assembly is subsequently separated from the molding tool. The curing of the surface coating material in the clearance then takes place substantially under atmospheric pressure. It was shown, surprisingly, that curing under atmospheric pressure results in a particularly good quality of the surface coating.
The filling of the clearance with surface coating material advantageously takes place with particular uniformity in all the regions of the clearance if the surface coating material is introduced into the clearance by means of overpressure or underpressure. It can be provided that the surface coating material is pressed into the clearance. Alternatively, the clearance may be at least partially evacuated before or during filling. In the method according to the invention, however, an overpressure or underpressure is applied only during the filling of the clearance, while the curing of the liquid surface coating material takes place under atmospheric pressure.
The method can be carried out particularly cost-effectively when the molding tool inner wall facing the interior fitting surface to be coated is finished with an optical surface quality, for example polished with a high gloss. Further working of the surface of the interior fitting provided with a lacquer or resin layer may thereby be dispensed with.
It is beneficial if the molding tool inner wall facing the interior fitting surface to be coated is produced from high-grade steel, rustproof, acid-resistant, hardenable steel, silicon, ceramic or aluminum. Materials of this type have high resistance to the usually aggressive lacquers or resins which can be provided for surface coating. Since the curing of the liquid surface coating material takes place under atmospheric pressure and not under overpressure, even brittle ceramic material may be employed in order to form the inner wall.
In a particularly preferred embodiment of the method according to the invention, there is provision for the molding tool inner wall, facing the interior fitting surface to be coated, to be hard-chrome plated since particularly good resistance to lacquers and resins can be achieved thereby.
It is advantageous if the at least two components of the liquid surface coating material are mixed with one another homogeneously immediately before the filling of the clearance and are fed to the molding tool at an approximately constant flow velocity. Feeding the mixture of surface coating material as uniformly as possible prevents the formation of gas bubbles, so that a particularly good surface coating can be achieved without any after-working.
During curing under atmospheric pressure, any gas bubbles possibly present can rise up into a filling duct connecting the clearance to the connection of the molding tool to the mixing and feeding assembly or into a venting duct or riser, so that, after curing, the surface coating is not impaired by gas bubbles present. It is particularly advantageous, however, if the surface coating material is fed to the clearance so as to be virtually free of bubbles, since this ensures that a particularly homogeneous and uninterupted quality of the surface coating is achieved.
Synthetic resins on a polyurethane, polyester, acrylic or epoxy base may preferably be used as liquid surface coating material.
The curing of the liquid surface coating material introduced may be assisted in that sensitizers sensitive to UV radiation are admixed with the liquid surface coating material and, during curing, the liquid surface coating material is exposed to UV radiation. In this case, the UV radiation may be fed into the molding tool via light guides. There may also be provision, however, for using a radiation-permeable glass or plastic mold. A reduction in the curing times may also be achieved by exposing the molding tool to heat during curing. It is also advantageous if the surface coating material is exposed to electromagnetic radiation, in particular microwave radiation, during curing, so that the interior fitting with a cured surface coating can thereby be removed from the molding tool in a particularly short time.
As already explained, there may be provision for removing the coated interior fitting from the molding tool after curing and then leaving the interior fitting without any further surface working.
The method according to the invention is employed, in particular, for the surface coating of the visible face of the interior fitting. Preferably, however, there is also provision for also covering edge regions of the interior fitting with the surface coating material by virtue of a corresponding design of the molding tool. Moreover, there may be provision for arranging the interior fitting within the molding tool in such a way that a clearance is formed on all sides between the surface of the interior fitting and the inner wall of the molding tool. Coating of the interior fitting on all sides can be achieved as a result. If only the visible face of the interior fitting is to be coated, this is laid with its rear side facing directly onto the inner wall of the molding tool, so that a clearance between the interior fitting surface to be coated and the inner wall of the cavity is arranged only in the remaining surface regions. Particularly in the case of the interior trim of motor vehicles, it is usually sufficient to coat only the visible side, facing the interior, of the interior fitting, whilst its rear side can remain essentially uncoated.
It may also be envisaged to coat the rear side of the interior fitting partially with surface coating material by means of the method according to the invention.
As already described, the surface coating material is introduced in the liquid state into the clearance by means of the mixing and feeding assembly. The surface coating material comprises at least two components. In this case, there may be provision for one of the components to have a solid consistency, at least in the initial state, for example in the form of a granulate or powder. Thus, for example, in the case of accelerated curing by means of UV radiation, pulverulent sensitizers may be provided as one component of the surface coating material.
The invention described extends, of course, not only to the method described above, but also to interior fittings, in particular shaped wooden parts, which are produced using this method.